Haskell type error using inferred type

haskelltypes

I created a data type to hold basic user information and loaded it into ghci. I then used ghci to look at the new data types type signature. I copied the type signature from ghci into the source file and tried to reload the file. Ghci threw an error.

The code and error are below.

My question is, why is this throwing an error. I used the type that was inferred by ghci.

User :: Int -> String -> String -> String -> String -> User
data User = User { userID :: Int,
                   login :: String,
                   password :: String,
                   username :: String,
                   email :: String
                   } deriving (Show)

Prelude> :r User [1 of 1] Compiling
User ( User.hs,
interpreted )

User.hs:3:0: Invalid type signature
Failed, modules loaded: none.

Best Answer

You may declare the type of a value (such as a function), but you may not declare the type of a data type or of a data constructor using the type declaration syntax for values. In fact, you already are declaring the full type of the data type and data constructor when you define them, so there is no need for an additional type declaration. So just leave out the line User :: ...; that line is a syntax error because it User with a capital U (constructor) and only lower-case names (values) may have ascribed types.

Related Solutions

Python – What’s the canonical way to check for type in Python

To check if o is an instance of str or any subclass of str, use isinstance (this would be the "canonical" way):

if isinstance(o, str):

To check if the type of o is exactly str (exclude subclasses):

if type(o) is str:

The following also works, and can be useful in some cases:

if issubclass(type(o), str):

See Built-in Functions in the Python Library Reference for relevant information.

One more note: in this case, if you're using Python 2, you may actually want to use:

if isinstance(o, basestring):

because this will also catch Unicode strings (unicode is not a subclass of str; both str and unicode are subclasses of basestring). Note that basestring no longer exists in Python 3, where there's a strict separation of strings (str) and binary data (bytes).

Alternatively, isinstance accepts a tuple of classes. This will return True if o is an instance of any subclass of any of (str, unicode):

if isinstance(o, (str, unicode)):

Python – How to determine a Python variable’s type

Use the type() builtin function:

>>> i = 123
>>> type(i)
<type 'int'>
>>> type(i) is int
True
>>> i = 123.456
>>> type(i)
<type 'float'>
>>> type(i) is float
True

To check if a variable is of a given type, use isinstance:

>>> i = 123
>>> isinstance(i, int)
True
>>> isinstance(i, (float, str, set, dict))
False

Note that Python doesn't have the same types as C/C++, which appears to be your question.

Best Answer

Related Solutions

Python – What’s the canonical way to check for type in Python

Python – How to determine a Python variable’s type

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